Researchers make appeal to the public to help determine why UK harbor seals are in steep decline

The U.K. is home to two species of seal, the gray seal and the smaller harbor seal. In some areas of Scotland, harbor seal numbers are in decline or are at drastically depleted levels compared to 20 years ago.

SMRU aerial surveys revealed harbor seal numbers in The Wash (southeast England), the main population center in England, are ~25% lower than in 2018. This large embayment has regularly been home to about 5,000 harbor seals when surveys began in 1988 and as recently as 2018. University of St Andrews Sea Mammal Research Unit (SMRU) are undertaking a program of work to understand the decline. It's thought that larger gray seals are potentially a factor.

The U.K. is home to more than a third of the world's gray seals (Halichoerus grypus). Their increase, since the cessation of culls in the 1970s and the ban of hunting in the early 1980s, has been hailed as a conservation success story. Indeed, SMRU aerial surveys revealed that the number of gray seals in southeast England has risen over 20-fold in the last 20 years from less than 2,000 to over 40,000.

Surveys conducted by partner organizations indicate there may be a recent decline in the number of gray seal pups born on the U.K. east coast. The reasons are unknown but marine heat waves (2023 and 2024) and a disease outbreak are potential factors.

Dr. Debbie Russell, Deputy Director of SMRU, said, "The apparent drop in gray seal pups needs to be investigated further. SMRU will be continuing their aerial survey program in August 2025 to survey key gray and harbor seal haul-outs. SMRU are also scheduled to survey all major gray seal breeding colonies in Scotland and east England in 2025 to generate estimates of pups born across colonies.

"These will indicate the degree to which declines at some colonies on the east coast are offset by increases at others, and the nature of the declines—whether the associated females have died or are still alive but did not give birth. If there are area-wide declines, then the cause may have also impacted already compromised harbor seal populations."

Dr. Russell added, "SMRU are conducting a program of work to determine the cause of the southeast England harbor seal declines, which will be critical to informing potential population recovery. The key potential causes of such declines include interactions with gray seals, increased human development, toxins or disease, with multiple interacting factors being most likely."

SMRU uses various methods including the deployment of adapted mobile phone tags to track where seals from both species feed at sea (glued to the fur, these fall off in the annual molt); taking a suite of health measurements, similar to what would be taken at the doctors; and collecting and examining their poo to understand what seals are eating.

However, such studies can only tell them so much—the public is needed to fill in the gaps.

Some individual gray seal males kill, and partially eat, harbor seals and also gray seal pups. SMRU are investigating how many gray seals exhibit this behavior and how frequently it occurs. This will allow them to understand whether or not this may be significantly impacting any harbor seal populations.

To do this, they need images and videos (together with location and time) from the public to identify individual gray seal males. Please report such observations by emailing [email protected].

Likewise, a key part of the jigsaw puzzle can only be provided by dead seals. The SMRU needs more information on the number of dead seals washed up and on the causes of death.